Add CTM to the cached layers' key and reduce render target pingponging in layer pre-rendering

The CTM is required on the key so we will re-render when necessary.
The SkGpuDevice change ensures big layers in the midst of atlas-able layers don't cause a render target switch.

R=bsalomon@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/433553002